1. Field of the Invention
The present invention relates to an ink-jet recording apparatus, and more particularly to the ink-jet head having an electrostatic actuator therein.
2. Description of the Related Art
Ink-jet printers are advantageous in that noise is extremely low at the time of recording, high-speed printing can be made, the degree of freedom of ink is so high that inexpensive ordinary paper can be used, and so on. Among those ink-jet recording apparatuses, an ink-on-demand type apparatus in which ink drops are ejected only when recording is required has been the focus of attention because it is not necessary to recover unused ink drops.
In such an ink-on-demand type apparatus, as described, for example, in Japanese Patent Post-Examination Publication No. Hei-2-51734, or Japanese Laid Open Publication No. 1986-59911, a print head is constituted by: a plurality of nozzle openings arranged in parallel to each other to eject ink drops therefrom; a plurality of independent ejection chambers respectively communicated with the corresponding nozzle openings and each having walls one of which is partly formed to serve as a diaphragm; a plurality of piezoelectric elements respectively attached on the corresponding diaphragms so as to serve as electromechanical transducers; and a common ink cavity for supplying ink to the each of the ejection chambers. In such a print head, upon application of a printing pulse voltage to any one of the piezoelectric elements, the diaphragm corresponding to this piezoelectric element is mechanically distorted so that the volume of the associated ejection chamber and the pressure in the chamber increases instantaneously. As a result, an ink drop is ejected from the ejection chamber nozzle opening towards a recording sheet.
In the aforementioned structure of the conventional ink-jet recording apparatus, however, much labor and time are required for mounting such piezoelectric elements on the ejection chambers. The piezoelectric elements themselves are made by slicing off tiny portions of a suitable base material. Electrodes for driving the piezoelectric elements are then formed therein. Maintaining size and material uniformity here are critical in order to minimize distortion effects caused by piezoelectric element production scattering. In some cases, irregular elements will cause noticeable variations in ink drop ejection speeds among the ink jet nozzles, leading to undesirable smearing or underprinting in the resultant image.
Once suitable piezoelectric elements are manufactured, they are painstakingly attached to each individual nozzle chamber with an adhesive agent. Interposing such an agent between the substrate and the piezoelectric element serves as a semi-insulator between the substrate and the piezoelectric element, thus reducing the driving efficiency of the ink jet recording apparatus. This is turn requires stronger driving voltages and ultimately reduces the lifetime of the ink jet recording apparatus.
Finally, the latest printer designs demand high speed and high printing quality, which in turn increases the overall number of nozzle openings and increases the density of the ink jet head device. As discussed above, since a separate piezoelectric element is required for each nozzle, machining becomes less accurate and troublesome to implement, and results in a lower product yield and product quality.
Other than the above system in which the diaphragms are driven by the piezoelectric elements, there is a system in which the ink in the ejection chambers is heated as discussed in either Japanese Patent Post-Examination Publication No. Sho-61-59911 or Japanese Laid Open Publication 1986-59911. In this system, the ink in the ejection chambers is heated by a heating means to induce ink evaporation and generate gas bubbles within the ink. As the ink begins to boil, pressure from the bubbles inside the chambers build. Eventually, this pressure build-up will force ink drops to be released through the nozzles.
This heating system is advantageous in that the heating resistors can be formed of thin-film resistors of TaSiO.sub.2, NiWP or similar material created by spattering, CVD, evaporating deposition, plating or other well-known techniques. The system, however, has a problem in that the lifetime of the head itself is short because the delicate heating resistors are injured by repetition of heating/quenching cycles and microshocks produced by the breaking ink bubbles.
Inkjet heads in which the force of electrostatic attraction is used for the actuator are commonly used, and one type of such ink-jet head is described in, for example, JP-A-289351/1990. In this reference, the ink-jet head comprises a nozzle, an ink passage in communication with the nozzle, a diaphragm provided at one part of the ink passage, and an electrode provided in opposition to a diaphragm having an air gap therebetween. An electrical pulse is applied between the diaphragm and electrode to deform the diaphragm by means of electrostatic force, thereby ejecting an ink droplets from the nozzle.
With this type of ink-jet head, the actuator contains a vibration chamber, which comprises the diaphragm and electrode, and is exposed to the open air. As a result, dust and other airborne particulate have a tendency to be attracted to the actuator when the diaphragm is driven. This problem can be corrected by, for example, sealing the actuator; but when the actuator is sealed, air sealed inside the vibration chamber adds resistance to the electrostatic attraction of the diaphragm, and may inhibit sufficient electrostatic attraction for normal operation. When the electrostatic attraction of the vibration chamber decreases, this results in sufficient pressure being generated to reliably eject ink having high print quality. To ensure the reliable ink ejection, a higher voltage applied to the actuator would be needed.